Carbonation behavior of reactivated recycled concrete fines containing residual sand: Effects of Ca/Si ratio adjustment and activation temperature

The valorization of recycled concrete fines (RCF) into reactive binders offers a sustainable solution for mitigating construction waste and carbon emissions. However, embedded sand particles hinder phase development during thermal activation. This study explores thermally activated sand-containing RCF by adjusting calcium-to-silicon ratio via limestone addition to produce reactivated cementitious materials (RCM). Carbonation-cured RCMs were analyzed for phase evolution, microstructure, and strength. Results showed that higher activation temperature with Ca addition enhanced sand reactivity and mineral formation, transitioning from α′_H-C₂S and β-C₂S below 1000 °C to low-reactivity CS or C₃S₂ at 1200 °C. Carbonation curing of RCM activated at 1000 °C with 20 wt.% limestone addition yielded the highest mechanical performance by optimizing phase reactivity, carbonation efficiency, and pore refinement, while lower strengths in other groups stemmed from insufficient CaCO₃ and silica gel. Life cycle assessment showed a 61 % CO₂ reduction compared to Portland cement, which validates thermochemical tuning for closed-loop RCF recycling.